This invention relates to the casting of thin steel strip and has particular application to the construction of casting rolls used in twin roll strip casters.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls. The casting surfaces of the casting rolls are generally provided by outer circumferential walls provided with longitudinal cooling water passages to and from which water is delivered through generally radial passages in end walls of the rolls.
When casting ferrous metals the rolls must support molten metal at very high temperatures of the order of 1640.degree. C. and their peripheral surfaces must be maintained at a closely uniform temperature throughout in order to achieve uniform solidification of the metal and to avoid localised overheating of the roll surface. It has therefore been normal to form the outer circumferential wall of each casting roll as copper or copper alloy sleeve mounted on a central stainless steel arbour and provided with closely spaced longitudinal water flow passages supplied with cooling water through water flow ducts formed in the supporting arbour. Such a roll construction is disclosed in our co-pending Australian Patent Application PO8328. In that roll construction the water flow passages are formed by circumferentially spaced holes drilled through a copper or copper alloy sleeve mounted on a central stainless steel arbour. The ends of the holes are all plugged to seal the water flow passages and the water flow passages are interconnected in groups such that each group of circumferentially spaced passages forms a single continuous water flow channel for flow of water back and forth between the two ends of the roll in passing from one end of the channel to the other. This enables a very even temperature distribution to be achieved both circumferentially and longitudinally of each casting roll.
Although the roll construction disclosed in Application PO8328 makes it possible to achieve a very even temperature distribution over the casting roll surface, it has been found that there are roll distortion and movement problems caused by the differential expansion of the copper sleeve and the supporting stainless steel arbour. The wall of the copper sleeve expands to a slightly greater radius at the side where it is in contact with the casting pool as compared with its side remote from the casting pool so that the sleeve develops a non-circular, generally oval cross section. This causes some parts of the sleeve to lose firm contact with the arbour during each revolution. The extent to which this occurs can vary along the roll so that the points of firm contact can be at arbitrary and varying positions along the roll. When the sleeve contracts on leaving contact with the casting pool during each revolution it will tend to contract towards the firm contact points and since these can be at arbitrary varying locations the sleeve can be caused to move longitudinally. Accordingly, the sleeve not only floats on the arbour in radial directions to produce gap control problems but it also suffers arbitrary longitudinal movements with consequent side dam control problems.
The floating movements of the copper sleeves on the arbours also causes the centre line of the gap between the rolls to move laterally back and forth during casting. Generally one of the roll arbours is set to be moveable under a constant spring bias which determines the gap between the rolls during casting. However, if the centre line of the gap moves due to movements of the sleeves relative to the arbours the spring loaded arbour will also move. Accordingly, even though a constant spring bias may be maintained there will be constant movements of the spring loaded arbour and a shifting of the gap position leading to gauge variations in the cast strip ie. the thickness of the strip fluctuates continuously as it is formed.
The present invention enables the above problems to be overcome by providing a new casting roll construction in which there is no central supporting arbour, the casting surface being provided by a copper or copper alloy tube which is connected directly to a pair of stub shafts making use of fasteners fitted into cooling passage holes in the roll tube.